1. Field of the Invention
The invention relates to the field of electronic optics, and more particularly to optical electron beam lithographic systems.
In electronic optics, it is frequently necessary to project, on a plane called "image plane", the image of an object materialized in a plane of the electronic optics system conjugate with the image plane. At every point of the image, the incident trajectories are inscribed in a cone of semi-aperture ".alpha.". This angle ".alpha." should be related with the part used of the last lens called "objective" which cannot exceed certain limits without the risk of aberrations exceeding the required precision. Let us mention particularly:
spherical aberration, proportional to the cube of angle ".alpha.",
chromatic aberration, proportional to the angle ".alpha.".
2. Description of the Prior Art
To limit the angle ".alpha.", the most usual solution, particularly in electronic microscopy apparatus, consists in disposing a diaphragm D in the main plane the objective LO as shown in FIG. 1, the objective forming in the image plane Pi an image of the object situated in the object plane PO.
A closely related solution consists in disposing the diaphragm D between the object plane or the last conjugate plane of the image plane and the main plane of the objective, as shown in FIG. 2. This solution may be selected when the penumbra defined in the main plane of the objective by the diaphragm and the last but one image is negligible with respect to the shadow of the diaphragm in the main plane. Its advantage is to leave place for a deflection system situated between the diaphragm and the objective, which need not necessarily have a pivot point in the plane of the diaphragm. This solution is often chosen for electron beam lithographic systems.
Another solution described in the French patent application 81 17846, and shown in FIG. 3, consists in disposing the aperture limiting diaphragm D as close as possible to the source S for example close to a first lens L1 and to design the electronic optic system 1 so that this diagram is conjugate with the main plane of the objective LO. This solution has the advantage of limiting from the beginning of the trajectories, the current of the charged particles and therefore of reducing as much as possible parasite effects of coulombic nature known under the name of "Boersch" effect or space charge effect.
In the above mentioned patent, it is explained that this solution may be particularly advantageous for so called "variable rectangular" or else "formed beam" electron beam lithographic systems.
In the three above mentioned cases, there exists a linear bi-univocal relationship between the diameter of the diaphragm materialized in the system and the angle ".alpha.". Thus, if it is desired to vary the angle ".alpha.", because from one use to the other the required precision is not the same, it is necssary to change the diaphragm, either by dismantling the system or by using a mechanical device with several positions, each position corresponding to a different diaphragm disposed in the path of the beam. Such a mechanical device has two drawbacks:
the mechanical complexity introduced into the system,
the limitation of the number of values which may be attriuted to the angle ".alpha.", often only three values.
Furthermore, in such systems, it is necessary to determine the illumination of the object from the source;
Given a source of diameter d.sub.s which emits in a cone of semiaperture .beta., it is conventional to use a system of electronic lenses, L.sub.S, for projecting on the object to be illuminated an image of the source whose diameter will be G.times.d.sub.s, for a beam aperture .beta./G; we then speak of critical illumination. This illumination mode is shown in FIG. 4.
It is also conventional to use an electronic lens system so that, in the plane of the object to be illuminated, the image of the source is thrown back to infinity and that thus an image of the angular diagram of the source is projected on the object. We then speak of KOHLER's illumination. This mode of illumination is shown in FIG. 5.
Nothing obviously prevents intermediate solutions between these two typical illuminations from being chosen.